1. Field of the Invention
The present invention relates to a resistor-provided breaker, wherein resistor contacts for allowing making and breaking to be achieved through resistors are connected parallel to main contacts for allowing making and breaking to be achieved through no resistor, and more particularly to a resistor-provided UHV (Ultra High Voltage) breaker applicable to a UHV electric plant such as a substation in a million-volt power supply system line.
2. Description of the Related Art
There is a resistor-provided breaker in which resistor contacts for allowing making and breaking to be achieved through a resistor are connected parallel to main contacts for allowing making and breaking to be achieved through no resistor. In this breaker, first; power transmission lines are connected to each other through a resistor, and thereafter the resistor is disconnected from the circuit, thus completing closure of the circuit. This can minimize, even at the time of making an unloaded 500,000-volt-order line, a very high overvoltage which occurs when a breaker without resistors is used.
Among resistor-provided breakers of this type, an example of a 500,000-volt breaker will be explained with reference to FIG. 1. In a tank 1, sealing an insulation gas therein, two main contacts 2a and 2b are contained. These contacts 2a and 2b are connected to resistors 3a and 3b, respectively. The resistors 3a and 3b are connected to resistor contacts 4a and 4b, respectively. An operation mechanism unit 5 is provided under the tank 1, and connected to the main contacts 2a and 2b and the resistor contacts 4a and 4b via rotary links 6a and 6b, provided in the tank 1. The driving force of the operation mechanism unit 5 is transmitted to the main and resistor contacts 2a, 2b, 4a, and 4b to make and break them, after the direction, in which the driving force acts, is changed by means of the rotary links 6a and 6b.
In the above-described 500,000-volt resistor-provided breaker, to make the circuit, the resistor contacts 4a and 4b are made about 10 msec. before the main contacts 2a and 2b. This operation minimizes an overvoltage which occurs at the time of making the circuit. After making the resistor contacts 4a and 4b, the main contacts 2a and 2b are made. On the other hand, at the time of breaking the circuit, the resistor contacts 4a and 4b are broke, before the main contacts 2a and 2b. In other words, in a resistor-provided breaker of this type, the resistor contacts 4a and 4b are operated before the main contacts 2a and 2b, at the time of both making and breaking the circuit. Therefore, the operation mechanism unit 5 can be made to have a simple structure using a spring mechanism, etc., and only one mechanism 5 can perform the operation.
Recently, it has been more and more requested that the length of a transmission line be increased, and that voltage supplied through a power transmission system line in order also be to increased in order to increase the transmission efficiency. To meet these requests, a million-volt-order (UHV) transmission system line is now being planned.
A resistor-provided UHV (for example, million volts) breaker has been proposed as a breaker for use in such a UHV transmission system line. This resistor-provided UHV breaker has a structure in which a resistor is connected parallel to a main contact at the time of breaking, so as to reduce the rate of increase in the transient recovery voltage which occurs at the main contact after breaking, and thereby facilitating the breaking operation. This structure can be employed in order to minimize the overvoltage (such as grounding) which occurs after breaking as well as at the time of breaking.
An example of a conventional resistor-provided UHV breaker for performing the above-described making/breaking of a transmission line with the use of a resistor will be explained with reference to FIG. 2. A main contact 2 is located in a tank 1 in which an insulating gas is sealed. A resistor 3 is connected parallel to the main contact 2. At the time of breaking, a resistor contact 7 is connected to the resistor 3 in series so as to break the resistor 3 30-40 msec. after the main contact 2. On the other hand, at the time of making the circuit, it is necessary to make the resistor contact 7 via the resistor 3 about 10 msec. before the main contact 2 so as to minimize the overvoltage, and then to make the main contact 2, as in the case of the aforementioned resistor-provided 500,000-volt breaker.
As described above, the resistor-provided million-volt breaker must perform such a very complicated operation that at the time of making the circuit, the resistor contact 7 must be made about 10 msec. before the main contact 2, and, at the time of breaking the circuit, be opened 30-40 msec. after the contact 2. To achieve the complicated operation of the resistor contact 7, the structure shown in FIG. 2 may be employed. The operation mechanism unit 5 is used to operate both the main contact 2 and the resistor contact 7. A delaying mechanism unit 8 is provided for delaying and advancing the making and breaking of the resistor contact 7, performed by means of the operation mechanism unit 5, respectively.
It is necessary to design the resistor-provided breaker shown in FIG. 2 with a sufficient allowance, since the erroneous operation of making the resistor contact 7, after the main contact 2, or of breaking the resistor contact 7, before the main contact 2, may damage not only the breaker itself but also the overall system employing the breaker as the result of the occurrence of a great overvoltage. However, elongating, in accordance with an increase in the allowance, the time period during which the current flow is continued increases the load on the resistor 3. As a result, the resistor 3 must be made large. To avoid this, it is desirable to design each structural element to have an appropriate allowance, so that it can have an appropriate size. However, if the range of variations in the delay time period, set at the time of making and breaking the main contact 2 and the resistor contact 7, is wide, the design allowance for each structural element must be increased.
Thus, it is important to accurately control the resistor contact 7 such that it is made and broke before and after the main contact 2, respectively (such control will be hereinafter called "UHV operation control"). For example: simplifying the structure of the operation mechanism can enhance the operation accuracy, since the more the structure of the operation mechanism is simplified, the more its reliability is enhanced. Consider here the structure shown in FIG. 2, wherein the making and breaking of the resistor contact 7 is performed by means of the operation mechanism unit 5 which also makes and breaks the main contact 2, with the timing of the breaking and making of the contact 7 being adjusted by the delaying mechanism unit 8. In this case, however, the delaying mechanism unit 8 has a complicated function for advancing and delaying application of a driving force from the mechanism 5 to the resistor contact 7. This function makes complicated the overall operation mechanism including the delaying and operation mechanism units 5 and 8, and makes it difficult to enhance the reliability of the mechanism by merely simplifying the structure.
Then, consider another structure which employs an operation mechanism unit dedicated to making and breaking the resistor contact 7 and an operation mechanism unit dedicated to making and breaking the main contact 2. If the aforementioned UHV operation control is performed with the use of these two operation mechanisms, it is possible that the overall operation mechanism not only has a complicated structure, but also performs inaccurate operations. This is because the UHV operation control is very hard to perform when grounding noise or the like is mixed into an instruction signal for causing the resistor contact 7 to make before and break after the main contact 2.